(1) Field of the Invention
This invention relates generally to transmission electron microscopes (TEM) and more particularly to means within a TEM for providing improved images of specimens.
(2) Description of the Prior Art
In a conventional TEM, the illuminating beam penetrates a given specimen area and is then focused by the lens system into a magnified image. To view such image requires the help of a mechanism by which the electron beam intensity in each picture element (pixel) can be optically perceived. Because of convenience and reduced cost, the conversion to an optical image for observation is usually accomplished by a fluorescent substance--such as cadmium or zinc oxide. These oxides are spread in a thin layer onto a support to form the viewing screen. This passive mode of visualization, while widely adopted--for lack of better alternatives--presents a number of limitations:
(a) the graininess of the fluorescent phosphor and diffusion of emitted light within the screen restricts high resolution observations to levels considerably poorer than achieved with photographic emulsions;
(b) the mechanism of light emission by electrons from current fluorescent substances is not very efficient, thus requiring substantial beam intensities for bringing screen images above perception thresholds. Especially when working at high magnifications and in the case of delicate biological preparations, this requirement is restricted by the damage induced in the specimen by the electron beam;
(c) as a consequence of electron interaction with the specimen, the effectiveness and contrast displayed by fluorescent screens decrease progressively when the electron energy increases; and
(d) poor screen visibility makes direct, high resolution focusing difficult at either high or low magnifications and thus leads to more than minimal specimen damage through prolonged radiation.
Thus, from what has been above-mentioned, the main difficulty in viewing and/or recording images, particularly in high voltage transmission electron microscopy, stems from the unavoidable fact that the pixels cannot yield information independently of each other because of crosstalk between neighboring elements produced by single or multiple scattering and by more copious radiative processes that affect the detectors (e.g., fluorescent phosphors).
More efficient alternatives to this mechanism aimed at eliminating some of the existing difficulties by taking advantage of the advanced electronic technology have been attempted and developed to various degrees of performance and satisfaction. For instance, image intensifiers have been used in which the global image is divided into a multitude of pixels which are detected and amplified individually and simultaneously for display on a final screen. However, since an intermediate screen image is still needed for the image intensification to be performed, most of the limitations described above are not avoided, and the overall performance remains relatively unsatisfactory. In addition, since existing image intensifiers are both delicate and sensitive to X rays, they cannot be placed directly in the electron beam because of radiation damage and high x-ray background smearing of whatever resolution might otherwise be available.
Several other types of electron microscopes are well-known in the art and, to a certain extent, avoid some of the above problems inherent in TEMs. The scanning electron microscope (SEM) employs an electron pencil beam which is focused on the specimen and is provided with an image detector which senses secondary electron emission from the surface of the specimen. Scanning transmission electron microscopes (STEM) employ a pencil beam which is swept across the specimen with the signal being obtained below the specimen. Not only is it difficult to keep STEM beams stable and properly oriented, but such beams have a propensity to severely damage the specimens being examined.
Examples of such microscope systems are shown in U.S. Pat. Nos. Re. 39,500 to Hoppe; 4,044,254 and '255 to Krisch, et al.
It is an object of this invention to provide a TEM with an improved imaging system that enables pixel data to be collected directly from beam intensities without intermediate conversions to visible images and free of contrast-limiting pixel interaction.
It is another object of this invention to provide an improved TEM imaging system wherein a direct specimen transmission profile can be exhibited on a Cathode Ray Tube monitor.
It is also an object of this invention to provide an improved TEM imaging system wherein pixel data for global two dimensional images can be obtained directly in digital form for instant storage or manipulation.
Still another object of this invention is to provide a TEM with the means for sequentially acquiring the image of a specimen in addition to its standard image acquisition capability by which all pixels are recorded at once.
It is still a further object of this invention to provide an improved TEM imaging system which can be added to a conventional TEM without significant redesign.